1. Field of the Invention
The invention relates generally to the field of communications, and more particularly, to systems for identifying and tracking radio frequency transmission signals and for positioning an antenna toward targeted signals.
2. Description of the Related Art
Depending on the application, one of several types of antennas can be utilized to implement a radio frequency (RF) link for a wireless communication system, wherein the RF link may transmit and/or receive audio, encapsulated data, compressed video, or other data. Types of antennas that may be used include omni, sector, and directional antennas. Those skilled in the art will understand that an omni antenna may radiate energy, for example, RF energy, approximately in, and receive energy approximately from, all directions (e.g., in a 360 degree azimuth). Those skilled in the art will also understand that a sector antenna may radiate or receive a cone of energy that is generally approximately between 50 and 120 degrees, and a directional antenna may radiate or receive a beam of energy within a much narrower angle in a determined direction with respect to the antenna. Directional antennas may have an angle of signal reception or transmission (i.e., a beam-width) that is less than that of a sector antenna and which is determined by the specific configuration of the directional antenna. The beam of energy transmitted or received by certain directional antennas may be referred to as a pencil beam because of its relatively narrow width as compared to the energy radiated by other types of antennas. Both sector and directional antennas need to be pointed, either manually or automatically, towards a target receive system or a source transmit system, as their beam-widths are less than 360 degrees. Directional antennas specifically require the most care as their beam-widths are typically less than about 10 degrees and in some cases less than about 1 degree.
Those skilled in the art will understand that the above antenna descriptions apply to both antennas used in transmit systems, as well as antennas used in receive systems. Many antennas can be used as either a transmit antenna or a receive antenna, or both, as in the case of a bi-directional link.
Between the output of a transmit antenna and the input of a receive antenna, the RF signal propagates through the air getting attenuated and bounced off terrain, buildings, and/or water. In order for a receive system to receive a desired signal, the signal typically must have enough power from the transmitter and gain from the receiver to overcome the attenuation due to air and satisfy the threshold signal level required by the receiver. In addition, the receive system must generally overcome natural and unnatural multi-path. Natural multi-path, which consists of bounced signals taking paths of varying lengths to get from the transmit antenna to the receive antenna, presents multiple images of the same signal at the receiver. Unnatural multi-path consists of undesired transmitted signals of the same, or similar, frequency and power levels as the desired signal. Unnatural multi-path may be an issue if multiple users are transmitting over the same, or similar, frequency simultaneously. The increasing prevalence of air to ground wireless communication, high-speed video, and data transmission is resulting in an increase in unnatural multi-path. In many areas of the world, environments are saturated in RF transmissions, thereby causing widespread interference.
Using an antenna with a narrowed beam-width may be required to minimize interference, as a narrowed beam-width corresponds with increased gain. Omni antennas generally have gains in the region of about 2 to 10 dBi (dBi refers to the relative gain/directivity of an antenna with respect to an equivalent isotropic antenna, which isotropic antenna radiates in all directions equally, expressed on the decibel logarithmic scale). Sector antennas generally have gains in the range of about 10 to 16 dBi. Directional antennas with beam-widths of less than about 10 degrees generally have a gain greater than about 20 dBi.
Selecting a receive antenna with a narrowed beam-width, for example a directional antenna, will generally allow a signal to be received from a greater distance, increase the strength of the received signal, and increase the resultant signal-to-noise ratio. The use of directional antennas, however, may limit the azimuth of signal reception since the beam-widths are typically less than about 10 degrees, and in some cases, less than about 1 degree. Careful positioning and continual adjustment of such antennas may be necessary to ensure proper signal reception. Presently, such positioning and adjustment is generally slow and often necessitates laborious input by a trained operator. These limitations may make directional antennas prohibitively cumbersome to use, particularly if the corresponding transmit or receive antenna is located on a moving device.